If the language only has trapped errors, which are indicated by seg faults, it's a safe language.
Every language has such errors. What does divide by zero do?
What does blowing the stack / infinite recursion do in Rust? It seg faults.
The seg fault is the safe behavior. If the stack overflow overwrote heap data structures or global data structures and the program kept running, that would be unsafe.
Returns a value, of course! (And that said, JavaScript does have some trapped errors, such as (1/0).foo.foo (and yes you need the second .foo…))
IMO, the execution "error" here (in this thread) is accessing memory illegally. Sometimes the runtime traps it, but sometimes it does not, and "sometimes" isn't always, so its effectively untrapped as we cannot depend on the trap. (Especially in adversarial circumstances.)
And further, the original quote talks about languages — the behavior of a language like C is that memory access is not necessarily trapped; the behavior is not well defined. Given the lack of a requirement in the C language for a trap, I think it is fair to call C "unsafe" given the above definition of safe/unsafe.
> What does blowing the stack / infinite recursion do in Rust? It seg faults.
Somewhat interestingly, it detects it and SIGABRTs, which technically isn't a segfault. And that's now some black magic that I'm curious about as I really thought it would have segfaulted.
I don't understand the nuance here. In my Firefox developer tools, I can do the following:
> (1/0).foo.foo
---> TypeError: (intermediate value).foo is undefined
> (1/0).foo
---> undefined
> (1/1).foo.foo
---> TypeError: 1.foo is undefined
> Infinity.foo.foo
---> TypeError: Infinity.foo is undefined
> undefined.foo
---> TypeError: undefined has no properties
While these are all different errors, I don't really understand why the '(1/0).foo.foo' case is an example of a _trapped_ error, but the others are not.
In Python, Java, C, OCaml, and most other languages, 1 / 0 aborts the program. That is, division by zero is a trapped error.
In JavaScript, it's not. It keeps going and lets you do stuff like access nonexistent properties .foo on the result, which are also untrapped errors.
So JavaScript is unsafe in Cardelli's terminology. It gives you untrapped errors rather than trapped ones. The program keeps chugging along until you find out later and have to trace backwards to the bug.
Prelude> 1.0 / 0.0 :: Double
InfinityTry it again with div for integer division.
I’m sort of surprised by that since my memory is that infinity isn’t a real number, rational, etc. That is, does infinity in Haskell obey some algebraic laws?
However integer division by 0 isn’t. div 1 0 will fail.
This answer has an interesting way of looking at it. If you go on the theory that floating points are supposed to represent reals, then in floating point, you can't tell if a value is actually zero or just indistinguishably close to zero.
In the case of "indistinguishably close to zero", you're getting the wrong answer, and the program doesn't halt. It keeps on chugging doing bad math. So that's an untrapped error, and it's UNSAFE by Cardelli's definition.
https://cs.stackexchange.com/questions/82811/why-do-floating...
The key point is that "safe" sometimes means "crashes" and sometimes means "doesn't crash". It's an auto-antonym in that sense.
A broader definition is "errors are flagged as early as possible", including with seg faults / hardware exceptions.
It's a signal handler, of course: https://github.com/rust-lang/rust/blob/d8bdb3fdcbd88eb16e1a6...
I don't think anyone would disagree with this. What they're saying is that a segfault is safe and that's because a segfault is essentially your OS's version of an out-of-bounds error, one of the reasons that C is not safe is because an out-of-bounds access will not necessarily cause a segfault.